A variety of tensioning systems have been devised for imparting a desired amount of closing force to a bolted flange arrangement. For example, a nut may be tightened onto a bolt with a predetermined amount of torque. The accuracy of this method depends upon knowing the amount of friction developed between the nut and the underlying surface, among other factors.
Hydraulic tensioning systems are used to apply a predetermined amount of tensile force to a bolt, with a nut then being positioned on the bolt to maintain the tensile force after the hydraulic tensioning system is removed. The amount of tensile force may be determined by measuring the pressure in the hydraulic system or by measuring the elongation of the bolt. The reactive load applied by a hydraulic tensioning system is carried to the underlying flange surface through a stand that surrounds the bolt and nut. Once the bolt is stretched to a desired tension, the nut is turned down onto the flange with a predetermined amount of torque. Access to the nut is provided through windows formed in the stand. When the tensioner is depressurized, the compressive load in the stand is transferred to the nut. This process may be repeated a second or more times to ensure that the proper amount of pre-load is maintained in the bolt because the nut may “settle in” when it first receives the compressive load from the stand, thereby somewhat relaxing the pre-load on the bolt. Such systems are available through Hydraulics Technology, Inc. (www.htico.com)
In applications where there is insufficient room to use a hydraulic tensioning system, a special hydraulic nut may be used. A hydraulic nut includes an internal piston/cylinder arrangement that allows the nut to expand axially in response to an applied hydraulic pressure, thereby tensioning the engaged bolt. A mechanical portion of the nut is then tightened to hold the nut in its expanded condition after the hydraulic pressure is removed. Such devices are expensive and may depend upon highly precise metal-to-metal seals for high temperature applications. Such devices are available through Riverhawk Company. (www.riverhawk.com)
An alternative to the hydraulic nut is the jackbolt tensioner. A torque nut is applied to a hand-tight condition. A plurality of jackbolts are threaded through the torque nut to push the torque nut away from the flange surface to tension the bolt. A hardened washer is placed between the jackbolts and the flange surface to protect against harmful load concentrations. Such devices are available through Superbolt, Inc. (www.superbolt.com)